268
ACUS03 KWNS 211929
SWODY3
SPC AC 211928

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0228 PM CDT Fri Aug 21 2026

Valid 231200Z - 241200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SUNDAY INTO
SUNDAY NIGHT ACROSS PARTS OF THE NORTHERN ROCKIES INTO NORTHERN
GREAT PLAINS AND ACROSS PARTS OF THE NORTHEAST...

...SUMMARY...
Strong thunderstorm activity posing at least some risk for severe
hail and wind appears possible across parts of the northern Rockies
and Great Plains, and across parts of the Northeast, Sunday through
Sunday night.

...Discussion...
Models indicate that prominent mid/upper ridging centered across the
southern Rockies through southern Great Plains will be maintained
through this period.  However, initially amplified ridging extending
through the westerlies to the north of this feature is likely to
become suppressed by a notable mid-level short wave trough, slowly
progressing east-northeast of the Canadian/northern U.S. Rockies
through the southern Canadian Prairies and adjacent international
border vicinity.

Downstream, large-scale mid-level troughing is forecast to only make
very slow eastward progress east of the Mississippi Valley through
Atlantic Seaboard/western Atlantic vicinity, while an evolving
embedded low slowly shifts eastward near/north of the lower Great
Lakes region, in response to at least a couple of shorter wavelength
perturbations pivoting around it.

In lower-levels, the primary surface cyclone is forecast to migrate
northward toward James Bay, with a weaker secondary low perhaps
migrating inland across parts of the northern Mid Atlantic/southern
New England, before weakening further.  It appears that the
composite leading edge of cooler low-level air trailing this
feature, will stall and weaken across parts of the southern Great
Plains and Gulf Coast into southern Mid Atlantic.

...Central and eastern Montana...
There remains a signal in latest model output that forcing for
ascent within the base of the progressive short wave trough will
support increasing thunderstorm development across the higher
terrain of central Montana by late afternoon.  Coincident with
strengthening westerly mid-level flow to 30-40+ kt, and the
evolution of a strongly heated and fairy deeply mixed downstream
boundary-layer (with at least modest CAPE in excess of 1000 J/kg),
there appears potential for the evolution of an upscale growing and
organizing cluster into early evening.  If this occurs, the
environment appears conducive to an accompanying swath of strong to
severe gusts across parts of central into eastern Montana by late
Sunday evening.

...Dakotas...
Forcing aided by lower/mid-tropospheric warm advection downstream of
the progressive short wave trough may become increasingly conducive
to upscale growing thunderstorm development from the west/central
Dakotas state border through central/eastern South Dakota Sunday
evening into Sunday night.  Aided by inflow of seasonably moist
boundary-layer air supportive of CAPE on the order of 2000 J/kg,
there appears potential for activity to become organized with
potential to produce a swath or two of strong to severe gusts.  The
21/12Z NAM, in particular, generates a fairly notable convective
perturbation in association with this development.  If this signal
becomes supported by more model output in later runs, severe
probabilities may need to be upgraded further.

...Northern Mid Atlantic into southern New England...
Forcing for ascent aided by an initial short wave impulse pivoting
around the southeastern through eastern periphery of the Great Lakes
mid-level low probably will provide support for increasing
thunderstorm development across parts of eastern New York and
Pennsylvania into southern New England during the day Sunday.
Inland destabilization and vertical shear across this region remains
a bit uncertain, but it possible the environment could become
supportive of strong storms with potential to produce severe hail
and wind.

..Kerr.. 08/21/2026

$$